Science Objectives for EveryoneInternational Space Station Zero-Propellant Maneuver (ZPM) Demonstration shows for the first time new technology which rotates the Station by not expending on-orbit propellant.

Science Results for Everyone
No more million-dollar maneuvers. When the space station must rotate for operations such as docking of resupply vehicles, it uses thrusters that run on propellant costing nearly $10,000 per pound. This demonstration successfully rotated the station 90 and 180 degrees without propellant, saving more than 1 million dollars worth of propellant on the 180-degree maneuver. The new technology uses gyroscopes, or spinning momentum-storage devices powered by solar energy, to maneuver along special attitude trajectories. It will substantially reduce propellant use and contamination of solar arrays and loads. With this technology, long-duration space exploration missions can carry less propellant and more provisions.

The following content was provided by Nazareth Bedrossian, Ph.D., and is maintained in a database by the ISS Program Science Office.

The capability to rotate the ISS is necessary for assembly and operation, e.g. reorientation of the ISS for Space Shuttle and Russian resupply vehicles docking. Rotational maneuvers are performed using ISS thrusters which consume precious propellant at a cost of nearly 10,000 dollars per pound.

The goal of Zero-Propellant Maneuver (ZPM) is to reduce the cost of operating the ISS and provide a backup capability to thrusters. The ZPM is a new attitude control concept that was developed to take advantage of the orbital environment, i.e. gravity, aerodynamics, to perform the maneuvers with the ISS Control Moment Gyroscopes (CMGs) without exceeding their limits. This is similar to the way a sailboat would tack against the wind. ZPM will also reduce propellant use when used in combination with maneuvers using only thrusters.

Compared to thruster control, the advantage of ZPM is to substantially reduce ISS lifetime propellant use, and avoid solar array contamination and loads. Further, this technology will be even more valuable for future human exploration of the solar system as the propellant savings will allow for increased payload or provisions.

Description
To maintain its orbit and perform necessary attitude maneuvers, the ISS is equipped with thrusters and Control Moment Gyroscopes (CMGs), i.e., spinning wheel momentum-storage devices which are powered by solar energy. Small attitude adjustments can be accomplished with CMGs. However, large-angle rotations carried out with the flight software require more momentum than CMGs can provide, resulting in saturation. For this reason, thrusters are used to rotate the ISS. But thrusters consume precious propellant and their plume can contaminate and stress the solar arrays.

To perform rotations, the ISS flight software uses an eigenaxis trajectory, which is the shortest-distance kinematic path. Most spacecraft use this approach as it is simple to implement in flight software. According to Euler's rotation theorem, any two orientations are related by a common axis, the eigenaxis, about which rotation by a specific angle, the eigenangle, accomplishes the transition from one orientation to the other. To follow the eigenaxis trajectory, the attitude control system must overcome inertial and environmental dynamics, such as torques due to gravity or aerodynamics. As a result, CMGs reach momentum capacity even if maneuvering at a slow rate.

The Zero Propellant Maneuver (ZPM) concept is based on developing a special non-eigenaxis attitude trajectory that takes advantage of the nonlinear system dynamics to complete the maneuver without the need to use thrusters. The attitude trajectory modulates the attitude-dependent environmental torques acting on the ISS to maintain the CMGs within their capacity limits. ZPMs can be used to perform rotational state transitions (attitude, rate and momentum), which can be either a maneuver between prescribed states and/or an attitude maneuver used to reset the CMGs. The ZPM attitude trajectory is generated by formulating the ISS maneuver as an optimal control problem. It is solved using DIDO, an optimal control software package developed by Professor Mike Ross at the Naval Postgraduate School. ZPM will also reduce propellant use for spacecraft maneuvers using only thrusters.

Space Applications
The ZPM concept can substantially reduce ISS lifetime propellant use while avoiding thruster plume loads and contamination of solar arrays. A reduction in propellant use not only saves money, but increases payload capacity for resupply vehicles. Even more importantly, ZPM provides the only means of control if ISS thrusters are unavailable. ZPM will also reduce propellant use for spacecraft maneuvers using only thrusters. This technology will be even more valuable for future human exploration of the solar system as the propellant savings will allow for increased payload or provisions.

Operational Requirements and Protocols
ISS operations should be approximately similar to those assumed when designing the ZPM trajectory. These include location of Mobile Transporter, rotary joint operations, and orbital position at which to start the ZPM.A ZPM trajectory is generated on the ground with the required attitude and attitude rate commands to maneuver the ISS between pre-specified attitude, rate, and momentum states. The ZPM commands (i.e. maneuver trajectory and timeline) must accommodate ISS Power, Thermal, Communication, and other system requirements. ADCO converts the ZPM commands into time-tagged command packets for uplink to the ISS Command and Control computers. The ZPM is executed by commanding the CMG attitude hold controller with the time-tagged commands. ADCO must monitor the maneuver from the ground for contingency action.

The ZPM concept was successfully demonstrated on the ISS. On November 5, 2006 and March 3, 2007 the ISS was rotated 90 degrees and 180 degrees, respectively, without using any propellant.

The 90-degree maneuver of ISS Stage12A was completed in 2 hours and reached 70% of CMG momentum capacity (Bedrossian, AIAA, 2007). The 180-degree maneuver of ISS Stage 12A.1 was completed in 2 hours and 47 minutes and reached 76% of CMG momentum capacity (Bedrossian, International Symposium on Space Flight Dynamics, 2007). The same 180-degree maneuver was performed with thrusters on January 2, 2007 and consumed 50.8 kilograms or 112 pounds of propellant. At an estimated cost of $10,000 per pound, the 180-degree maneuver with ZPM saved $1,120,000 (Kang, Society for Industrial and Applied Mathematics News, 2007).

The flight results were documented and compared to predicted performance. The data documented included attitude, momentum, and gimbal rates during the maneuver. Flight reconstruction was performed to resolve discrepancies between predicted and flight results.

The impact of this new technology is to substantially reduce ISS lifetime propellant use, and avoid solar array contamination and loads. Future applications that can also be performed non-propulsively include maneuvering the ISS to unload accumulated CMG momentum, recovering attitude control when CMGs are saturated, and recovering attitude control in the event of a tumbling ISS. Since ZPM will also reduce propellant consumption for maneuvers using thrusters, it can also be used for future long-duration space exploration missions where propellant savings are even more valuable than for ISS and will allow for increased payload and provisions. (Evans et al. 2009)

Ross IM, Fahroo F.
Legendre Pseudospectral Approximation of Optimal Control Problems. Heidelberg: New Trends in Nonlinear Dynamics and Control and their Applications. Lecture Notes in Control and Information Science; 2003.

Naval Postgraduate School Professor of Mechanical and Astronautical Engineering I. Michael Ross briefs NPS Space Systems Academic Group students on NASA's use of his optimal control software to maneuver the International Space Station cost-free, without the need to use thrusters and expend valuable fuel. US Navy Photo by Javier Chagoya.+ View Larger Image